High voltage semiconductor devices employ a variety of techniques to reduce surface breakdown effects limiting performance of such devices. Edges of doped regions tend to have cylindrical or spherical shapes resulting in higher electric fields being formed under reverse bias in portions of the depletion region at semiconductor device surfaces. These higher fields often result in avalanche breakdown at substantially lower voltages than expected based on the electric fields associated with the remainder of the device.
A variety of techniques have been developed to control the electric field geometry in depeletion regions at p-n junction edges. These include beveling device edges, in areas forming depletion regions, by chemical and/or mechanical techniques or creating one or more "floating field rings" or junction termination extension techniques. Beveling techniques can be labor intensive and require large depletion regions (circa 100 micrometers) in order to place the beveled region in coincidence with the depletion region with sufficient accuracy to be effective. Floating field rings increase breakdown voltages but require substantial area and do not achieve breakdown voltages expected for the bulk of the p-n junction. Junction termination techniques relying on (usually laterally) graduated doping provide near-ideal avalanche-limited breakdown voltages but tend to be implemented using photolithographic and doping techniques susceptible to processing variations which can reduce yields and/or provide sub-optimal device performance.
What are needed are methods and apparatus for providing high voltage devices having improved breakdown voltages and acceptable area requirements and which are realized through robust processing techniques.